The present invention relates in general to the processing of partial oxidation gas produced by gasification of fuels in a gasifier at a temperature above melting point of slag and containing finely divided impurities. The crude gas upon its discharge from the gasifier is normally cooled in an indirect cooling system whereby the exchanged heat is used for the production of steam and upon cooling dust particles are separated in one or more dust separating stages. The cooled and cleaned gas is desulphurized and supplied in the combustion chamber of a gas turbine of a combined gas-steam turbine power plant.
In the gas processing method of this kind, it is conventional to feed high temperature partial oxidation crude gas discharged from the gasifier into an indirect cooling system generating waste heat steam which is completely or partially utilized, upon a corresponding overheating if necessary, in steam turbines of the combined gas-steam turbine power plant. The cooled partial oxidation crude gas upon dust separation and desulphurization is fed at a temperature between 600.degree. to 700.degree. C. in the combustion chamber of the gas turbine of the power plant. In this prior art method it is known how to prevent the obstructions in the waste heat boiler caused by sintering of dust particles entrained in the coarse gas stream, namely by solidifying the powdery impurities before their entry into the waste heat boiler by corresponding cooling of the crude gas exiting from the gasifier. In conventional methods, this gas cooling or quenching has been made, for example by sprinkling the crude gas with cold water.
In the German Patent DE-PS No. 24 29 993 a method is described in which the cooling of the crude gas is performed by a partial stream branched from the indirectly cooled gas stream. The branched partial stream is returned into the crude gas between its outlet from the gasifier and its inlet in the waste heat boiler.
Both prior art cooling methods however have certain disadvantages. In quenching by means of water the consumption of waste water is of necessity increased and in considering environment protection regulations which tend to become more and more rigorous, the antipollution measures become more expensive. In quenching by means of the returned gas stream it is always necessary to give outlets to the entire enthalpy of the produced crude gas stream between the gasifier and the reception point of the returned partial stream of gas through the indirect cooling in the waste heat boiler. Accordingly, a corresponding dimensioning of the heat exchanging surfaces in the waste heat boiler is needed.
The procurement and operation of such a waste heat boiler, however, is expensive inasmuch as large heat exchanging surfaces of high temperature and corrosion resistant material are necessary, whereby with decreasing gas temperature at the outlet from the waste heat boiler the requirements put on heat exchanging surfaces increase disproportionally. In addition, the operation of the waste heat boiler becomes more expensive due to the fact that the heat exchanging surfaces are subject to a permanent soiling by impurities contained in the crude gas and must be periodically cleaned by soot blowers or by acoustic devices. In the intervals between two cleaning periods the steam producing efficiency of the boiler deteriorates proportionally with the increased soiling.
Since the produced partial oxidation gas is to be processed in subsequent gas-steam turbine power plants, it must be taken also into consideration the fact that according to the contemporary state of the art the steam turbine process has normally a worse degree of efficiency than the gas turbine process. In order to compensate this difference, it is necessary to overheat at a high pressure the waste heat steam generated during the cooling of the partial oxidation has and accordingly this contributes to a further increase of operational costs of the waste heat boiler.